1. Field of the Invention
The present invention is in the field of kitchen ventilating systems, and more particularly is a system for removing cooking fumes from a kitchen where such fumes are produced in large quantities, as in a restaurant.
2. The Prior Art
A number of techniques have been used to dispose of the fumes produced in large-volume indoor cooking operations. Because the fumes are airborne, it is usual to urge the fume-laden air into an exhaust hood or plenum through which it is conducted to the outdoors, normally by the assistance of an exhaust blower. The main challenge is to capture all of the fume-laden air to prevent its escape into the kitchen, and to accomplish this end with a minimum expenditure of energy.
There are two main ways in which energy is expended. The first way is in moving the air within the ventilating system. It is well known that the energy expended is approximately proportional to the amount of air moved, and increases rapidly if higher velocities are required. Thus, if energy expenditure is to be minimized, excessive air velocities and excessive volumetric flows should be avoided.
The second main way in which energy is expended is by the undesirable discharge of conditioned air; that is, air which has been heated, cooled, filtered, humidified or dehumidified to make it comfortable to persons inside the building. When fume-laden air is removed from the building by the exhaust system, the air removed must be replace, or made up for, by bringing an equal volume of "make-up air" into the building. It would be ideal if the stove ventilating system could operate with only unconditioned make-up air, so that none of the more expensive conditioned air would be blown out the exhaust. In practice this ideal has not been attained, and typically 40 to 60 percent of the air discharged in the exhaust is made up of conditioned air, in a well designed installation. The degree to which the ideal is approached depends on the design of the system, which in turn must respond to numerous other constraints and desiderata which in some instances may conflict with the goal of minimizing the discharge of conditioned air. Some of these other constraints and desiderata are: the physical layout of the kitchen; the requirements of the building code, intended to promote health and safety; the margin of reserve provided to enable the system to cope with overload conditions; ease of maintenance; comfort of the cook; installation cost; and, cost of operation.
One approach to the design of the stove ventilatng system is shown in U.S. Pat. No. 3,292,525, issued Dec. 20, 1966 to Jensen. Jensen locates a make-up air outlet adjacent the front edge of the cooking surface and blows through the outlet a blanket of unconditioned air directed rearwardly and upwardly toward the mouth of an exhaust hood. The blanketing air must be discharged through the outlet with a relatively high velocity to assure that the horizontal velocity component of the air at a point over the cooking surface will be roughly comparable to the vertical velocity component of the fumes at that point. To produce such a high velocity discharge requires considerable energy.
The high velocity air curtain necesarily produces a venturi effect which causes conditioned air from the kitchen to be drawn into and mixed with the air of the curtain, and exhausted into the outdoors, constituting a waste of energy.
Further, the curtain of unconditioned air blows over the hands and arms of the cook, and if the unconditioned air is sufficiently cold the wind-chill factor could cause discomfort to the cook. A similar arrangement is shown by Jensen in U.S. Pat. No. 3,260,189 issued July 12, 1966 and in U.S. Pat. No. 3,386,365 issued June 4, 1968.
A different design approach is shown by Jensen in U.S. Pat. No. 3,400,649 issued Sept. 10, 1968. In that invention, a make-up air duct is located adjacent the rear edge of the cooking surface, and oriented to direct a blanket of air vertically upward. Fumes rising from the cooking surface are drawn toward the upwardly-moving blanket of air by the venturi effect. Although this approach does not subject the hands and arms of the cook to a chilling blast of air, it achieves this advantage at the expense of high energy consumption, because the venturi effect is relatively weak at distances of several feet from the upwardly-moving blanket, necessitating the use of a high velocity discharge. To the extent that the arrangement is effective in drawing fumes into the upwardly-moving blanket, it is also effective in drawing conditioned air from the kitchen into the blanket; consequently, energy is wasted as the conditioned air is exhausted to the outdoors.
This deficiency of the back-shelf ventilator of U.S. Pat. No. 3,400,649 is purported to be remedied by the arrangement shown in U.S. Pat. No. 4,089,327 issued May 16, 1978 to Welsh. As in U.S. Pat. No. 3,400,649, an outlet adjacent the rear edge of the cooking surface produces an upwardly-directed blanket of air toward which the fumes are drawn by the venturi effect. In an effort to prevent so much conditioned air from the kitchen from being drawn into the blanket of air and subsequently exhausted, a large hood is provided, extending forward beyond the front edge of the cooking surface above the cook's head. Within this hood a stream of unconditioned air is directed rearwardly from an outlet inside the front wall of the hood, toward the mouth of the exhaust plenum at the rear of the hood. Although this serves to dilute te exhaust air with unconditioned air, thereby increasing the percentage of unconditioned air, it does nothing to reduce the open space between the hood and the cooking surface, through which conditioned air is drawn from the kitchen. Welch's invention presumably would require greater blower capacity than the system shown in U.S. Pat. No. 3,400,649 for producing and exhausting the airstream within the hood.
From the above description of the prior art it would appear that although the arrangements described are noteworthy as representing different approaches to the use of unconditioned air for make-up, none of the arrangements altogether achieves the goal of capturing all of the fume-laden air with a minimum expenditure of energy.